Hypothalamic projections to the spinal segments and preganglionic regions by means of choleratoxin subunit B conjugated HRP in the chicken.

Hypothalamo-spinal projections were studied using the method of retrograde axonal transport of choleratoxin subunit B conjugated to horseradish peroxidase (CTb-HRP) in chickens. CTb-HRP was injected into the segmental and preganglionic regions of spinal segments 23-24 and segments 31-32 which are sympathetically and parasympathetically connected with the cloaca, respectively. Many labeled SP-neurons (segmental projection neurons) were found in the paraventricular nucleus (PVN), except in its lateral protrusion. The distribution of these labeled neurons continued to the more caudal areas such as the vicinity of the nucleus periventricularis hypothalami (PHN), areas medial to the stratum cellulare externum (SCE), the nucleus interstitialis (IS), and the posterior hypothalamic areas. Some labeled SP-neurons were also found in the lateral hypothalamic area. These labeled SP-neurons were distributed in almost the same areas in both cases of injection into segments 23-24 and 31-32. However, the number of labeled neurons was somewhat different rostrocaudally according to the spinal levels injected. In contrast, after microinjections into the spinal preganglionic regions, a small number of PP-neurons (preganglionic projection neurons) were found only in the ventral part of the PVN. These PP-neurons showed the rostrocaudally discernible segregation; PP-neurons in the rostral PVN and those in the caudal PVN were predominantly labeled by microinjections into segments 23-24 and segments 31-32, respectively. The present study suggests that the rostral PVN is more sympathetic and the caudal PVN is more parasympathetic in chickens.(ABSTRACT TRUNCATED AT 250 WORDS)